1. Field of the Invention
The present invention relates to a method for reducing processing power requirements of a video decoder, and more particularly, to a digital video decoders and the reduction of computational requirements of these decoders.
2. Description of the Related Art
Digital video has become widespread in the field of consumer electronics, due in large part to the emergence of digital video standards such as MPEG1 (see “CD11172—Coding of Moving Pictures and Associated Audio for Digital Storage Media at up to about 1.5 Mbps” by International Organisation for Standardisation, ISO MPEG Document, ISO-IEC/JTC1/SC2/WG11, 1994), MPEG2 (see “IS13818—Generic coding of Moving Pictures and Associated Audio” by International Organisation for Standardisation, ISO MPEG Document, ISO-IEC/JTC1/SC2/WG11, 1994), and the emerging standard MPEG4, which includes further added functionality and quality over MPEG1 and MPEG2. In MPEG1 and MPEG2 decoders, there are certain expectations regarding the decoder computational power which must be met in order to conform to the respective standard. For example, in the MPEG2 standard, Main Profile and Main Level (MP@ML) describes a set of bounds on the bitstream and associated video which all conformant MP@ML decoders must adhere to.
There are many situations where a decoder lacks sufficient computational power to decode a bitstream as it was intended. One example is on the internet where typically MPEG1 video bitstreams can be found and decoded using computers varying from high end scientific work stations to simple personal computers. When computational power is limited, one way the decoder can reduce it's computational requirements is to simply slow down the decoder output picture rate enough to permit all decoding to be accomplished. This was a very common method used by software based MPEG1 video decoders running on personal computers. Another method is to periodically skip the decoding of some pictures. For example, bi-directionally predicted pictures (B-pictures) can be skipped, effectively reducing the decoder output picture rate by picture-dropping. MPEG4 includes the concept of video objects, which are picture sequences which may be used to describe a separable object in a scene. If a scene contains many objects, the least important objects can be dropped in favor of giving the necessary computational power to the decoding of the more important objects.
There is a growing abundance of digital video bitstreams available, and a growing number of computational requirements associated with each. Due to the wide range of platforms that can be used for video decoding, there is a need for decoders which can decode bitstreams without requiring the full computational power required to meet conformance criteria. Existing techniques such as picture display rate reduction, picture dropping or object dropping techniques reduce the computational power at the expense of decoded video picture quality and synchronisation. For example, changing the picture display rate and picture dropping can have an effect on the perceived synchronisation of audio and video, commonly referred to as lip-synch. It also has the effect of making scenes with motion appear jerky.
An object of this invention is to permit video decoders to reduce the computational requirements of decoding a bitstream without affecting the picture rate. This will permit lip-synch to be maintained while compromising visual quality. It will also permit scenes which become jerky using picture dropping techniques to appear more smooth. Similarly, it will permit more objects to be present in the scene, since the computational power can be allocated to more objects, at the expense of object quality.